Field of the Invention
This invention relates to the field of radionuclide detection and characterization; and more specifically to the quantification of radionuclides by measurement of single-energy gamma rays in a fixed-geometry system.
Background of the Invention
A number of industries, including oil and gas production, rare earth mineral processing, and phosphate rock processing, generate waste contaminated with radioactive materials. These radioactive materials are generally referred to as naturally occurring radioactive materials (NORM) or technologically enhanced naturally occurring radioactive materials (TENORM). NORM and TENORM are not man-made radioactive materials and are not produced by splitting atoms in a nuclear reactor or by bombarding atoms with subatomic particles in accelerators. In the case of TENORM, the radionuclides may be enhanced by man, for example, by concentration of the radionuclides, but like NORM, TENORM is not produced by man. Some of the radioactive materials in NORM and TENORM may be subject to environmental regulations and require special disposal. NORM and TENORM may include radionuclides such as U-238, U-235, Th-232 as well as their respective progeny radionuclides (e.g., Ra-226, Po-210, etc.). As U-238, U-235, Th-232 and the unstable isotopes of their progeny decay; alpha, beta, and gamma radiation may be emitted. Radiation may continue to be emitted until a stable isotope in the chain of decaying progeny is reached, for example, Pb-206 is a stable isotope and the end product of the U-238 decay chain. The emitted radiation from the unstable radionuclides may be detected and used to calculate the concentration and activity of the unstable radionuclides, and more particularly, those unstable radionuclides which are subject to environmental regulation.
Present techniques for the detection of NORM and TENORM involve extracting samples from a suspect material (e.g., a waste) and then sending each individual sample to a laboratory for analysis, or performing gamma spectroscopy in the field. Laboratory analysis may be costly in both time and expense. For example, a typical laboratory analysis of a sample may take weeks and require expensive and rigorous sample processing. Further, during the time in which a sample is being analyzed, the remainder of the suspect material may be moved to and stored in a holding facility until the laboratory has completed the sample analysis. Also, as discussed above, each and every sample may be sent to the laboratory for analysis. Thus, any time a new waste material is produced or a NORM/TENORM survey is needed for any other reason, samples may be extracted and sent to the laboratory for analysis.
Alternatively, field analysis may be done using specialized equipment. For example, the activity of radionuclides may be measured using a multi-channel analyzer that scans a wide range of gamma ray energies, allowing for the identification and quantification of photons with energies corresponding to the applicable radionuclide progeny. This, multi-channel analysis for gamma radiation (gamma spectroscopy) may require careful sample preparation, and may include drying the sample, weighing a measured amount for analysis, and counting using a gamma spectroscopy system for a time ranging from several minutes to many hours. This technique is complex and may require an involved sample preparation procedure. Further, multi-channel analyzers used to capture multiple emitted gamma rays at once are expensive. Lastly, the operation of multi-channel analyzers is complex and may require trained personnel to operate in order to produce accurate measurements, for example, some radionuclides may emit radiation of an energy which obscures or may be confused with the emitted radiation of another radionuclide desired for analysis. Without proper training, it may be difficult to distinguish between the two activities of the radionuclides and thusly, an inaccurate measurement may be produced.
Consequently, there is a need for an improved radiation survey process for a practical cost-effective method of quantifying the amount of radionuclides in a waste sample in the field.